The objective of the present study was to investigate the removal of nitrogen in the form of ammonium ions (N-NH_4^+)from aqueous solutions using natural clay and zeolite. The Croatian zeolite clinoptilolite from the area of Donje Jesenje and Croatian bentonite clay from Kutina area were used as natural filtration materials . Alkaline and acid modification of natural clay was performed. The N-NH_4^+ removal efficiency by clays in a dry state as well as in liquid colloidal state have been investigated. The highest removal efficiency value for N-NH_4^+ (61.1 wt%) was achieved with the natural zeolite at the lowest used initial concentration, i.e. at a concentration of 100 mg N-NH_4^+/l. With the increase of the initial concentration of ammoniacal nitrogen, the removal efficiency quickly decreases. This is expected as zeolites and clays have limited sorption capacities. The practical aspect is important since an ammoniacal nitrogen elimination of more than 60.0 wt% for water containing less then 100 mg N-NH_4^+/l can be achieved. The ammoniacal nitrogen removal efficiency is higher for clays in a liquid colloidal state compared to the clays in a dry state. It has been found that the acid modification of the clay decreases the efficiency of ammoniacal nitrogen removal.